This invention relates to a rotational sensor for use with an in-vehicle navigation system, a navigation system that uses the sensor, and a vehicle with the sensor installed. The rotational sensor is created by placing two gravitational accelerometers configured at 90 degrees with respect to one another or otherwise out of synch and mounted at or near the center of a vehicle wheel or mounted at a given radius from the center of the wheel. As this resulting sensor is rotated, sine and cosine signals with a quadrature relationship are generated with respect to the earth""s gravity acceleration force vector, from which both rotation displacement and direction of rotation displacement can be determined. These signals may then allow the counting of the turns of the wheel, thus estimating the distance and the rate at which the vehicle has moved. A self-contained version of this device including a transmitter can relay this information to a receiving unit located within the vehicle. When one of these devices is located on each of the steerable wheels of the vehicle, the relative heading-direction of the vehicle may also be estimated.
Vehicle navigation systems require dead-reckoning information between GPS fixes. This becomes very important in urban environments and where the environment obstructs a line-of-sight view of the GPS satellite system, such as tree foliage which arches over the roadway, and in covered structures such as tunnels, garages and bridges. It is also important because of the error built into the prior art GPS signal for non-military uses. Often the vehicle has an existing speed pulse signal generated from the drive train or a wheel, intended primarily for the speedometer/odometer, the cruise control, or the anti-lock braking system. This signal will typically give one pulse per revolution, which at low speeds offers very poor resolution and poor update rates. It also offers no forward-reverse direction information. Forward-reverse direction information is generally provided by a switch closure indicating the vehicle is in reverse gear. No prior art used accelerometers engaged to a wheel to use measurement of gravity at various positions of the accelerometers during various stages of wheel rotation to provide indication and direction of wheel rotation of a vehicle.
In the case of xe2x80x98after marketxe2x80x99 equipment for the vehicle market, it is required to find the wire containing the signal and properly attach to it without disturbing the signal""s ability to do its original job. If an automotive mechanic or installer does the job, there is added cost.
In addition, the xe2x80x98after marketxe2x80x99 implementation of speed pulse is typically implemented via the signal that provides cruise control. In some cases this signal is unreliable at speeds under 20 mph, and on some vehicles is completely absent at speeds under 5 mph.
To date, neither a rotational sensor nor a vehicle navigation system in combination with a rotational sensor has been suggested which can provide a vehicle with accurate running fix information without a hard wired connection to the sensor using accelerometers while providing direction of wheel rotation as well as speed of rotation information.
As a result, a primary object of this invention is to provide a rotational sensor that provides an in-vehicle GPS navigation system with wheel rotation information from accelerometers engaged to a wheel to measure gravity at various positions of the accelerometers during various stages of wheel rotation to provide indication of wheel rotation. A second object of the invention is to provide a rotational sensor that provides direction of wheel rotation information to a vehicle navigation system. A third object of the invention is to provide a rotational wheel sensor that does not require a hard-wired communication path to a vehicle navigation system.
The rotational wheel sensor and a vehicle navigation system in combination with the rotational wheel sensor of this invention satisfy all of the above objects plus others not mentioned. The rotational sensor at a minimum will have two gravity-sensing accelerometers aligned ninety degrees relative to each other. The rotational sensor can be attached to a wheel of a vehicle, or preferably to each of the front steerable wheels, and provide speed-or distance-measured data. In one embodiment, this device can have its own power supply and use a radio frequency transmitter to communicate its digital data to a remote receiver unit to collect the data for the navigation unit requiring the dead-reckoning data.
When this device is used on each front wheel, a first sensor will mount on the left front wheel and a second sensor will mount on the right front wheel. In this case, the devices will provide the speed and distance information, but additionally will provide relative heading information. When the vehicle makes a left turn, the right wheel must travel a greater distance than the left wheel; this information may be converted into relative heading information by a computerized device.
Additionally, the ordinary prior art speed pulse system offers very low resolution. This invention offers very high resolution, down to a small fraction of a revolution of each wheel. This invention provides low speed and vehicle forward-reverse direction information and accurate vehicle heading information. Use of two gravity sensing accelerometers per wheel and the quadrature (90 degree shifted waveform) will allow the sensor of this invention to provide information as to the direction the wheel is rotating, i.e., giving information about the direction the vehicle is traveling (forward or reverse).
Each vehicle model will have its own specific wheel diameter. This diameter could be entered into the system. Or the navigation system can xe2x80x98learnxe2x80x99 the diameter by comparing the data received from the wheel sensors with other navigation information it will have as input data, such as GPS fix information or map and maneuver correlation. Over a short period of time, the system can discover or xe2x80x98learnxe2x80x99 the effective wheel diameters and can use the data this invention supplies to achieve very high accuracy.
It is necessary to insure that each wheel sensor is able to send its data and have it properly identified and not interfered with by simultaneous transmissions. Techniques such as assigning a unique manufacturer and serial number device address identifier of sufficient length so that the chance of misidentification of data from other vehicles or from the other transmitter on this vehicle is very unlikely. Also, due to the possibility of disruptions in the RF environment, error correction and data verification can be used. Further techniques include using a transmitter at each sensor having its own channel, spread spectrum radio techniques, or time-randomized transmission bursts in order to permit a system to operate with multiple wheel sensor devices and even when other nearby vehicles are equipped with the same type devices.
Furthermore, special care in choosing the formatting and inclusion of certain data can reduce the significance of missed data. Each sensor module maintains a clock and also a wheel position, which contains the positions up to some number of revolutions of the wheel, with a resolution of a small fraction of a wheel rotation. In this case, even if a data transmission is corrupted or otherwise missed, the receiver will, at the next transmission, have the true position of the wheel and the time stamp of that position.
Also the timestamp at the time of transmission of the data is sent so that the receiver can monitor the sensor clock to further increase the precision. This also allows the navigation system to coordinate the signals from this invention with other sensor information such as GPS.